Millipedes occasionally exhibit population explosions in a huge way. Parafontaria laminata in Japan becomes so numerous on occasions that they stop trains by causing the wheels to loose traction. Masses of millipedes have also been reported as stopping trains in Germany, France and Hungary. In West Virginia USA in 1949 one swarm of millipedes is estimated to have been made up of 65 million individuals.

Millipedes are generally considered to be harmless but they can be pests in some circumstances. In Europe sugar beet is the main crop to suffer from millipedes. In Africa Spirostreptids can be damaging to cotton and groundnuts. Blaniulus guttulata, the Snake-spotted Millipede accounts for most of the complaints against millipedes, these averaged about 20 per year in 1940, while in America Oxidus gracilis is the main culprit. In Australia Ommatoiulus moreleti, which was introduced in 1953, is a pest of crops and a nuisance in houses, which it invades at night because it is positively phototactic (attracted to light) in areas where it occurs.